The invention relates to a method for measuring the gas content of a fluid, which comprises gas bubbles, in which method microwave radiation is transmitted through the fluid.
The invention also relates to a measuring arrangement for measuring the gas content of a fluid, which comprises gas bubbles, the measuring arrangement being arranged to transmit microwave radiation through the fluid, and comprising a transmitting antenna, a receiving antenna and a microwave measuring device, which are arranged to measure at least one variable of the microwave signal as the signal travels through the fluid.
At present, the gas content and especially the air content of a fluid or a liquid substance are measured mainly by means of methods and devices based on the ultrasound and the measurement of density. Attenuation of the ultrasound is a function of the gas content of a fluid: the higher the gas content the greater the attenuation of the ultrasound. In the paper industry, the gas content of papermaking pulp is typically measured by means of the ultrasound. The quality of the final product, i.e. paper, depends on the quality of the liquid pulp, which, in turn, is partly dependent on the gas content thereof.
Finnish Patent 84,299 discloses an arrangement where the air content of a suspension is determined by measuring its water content at two different known pressures. Also American Patent 4,852,395 discloses an arrangement where the gas content of a flowing fluid is determined by measuring the transmissivity of microwave radiation through the fluid at two different known pressures.
The prior art also includes a gas measuring device comprising means for determining the gas content of a fluid based on changes caused by pressure variation in the variable of a microwave signal, This arrangement is described in greater detail in Finnish Patent 972,172, which is incorporated herein by reference.
However, a problem with gas content measurement based on the attenuation of the ultrasound is that the method cannot be applied in suspensions with a high percentage of solids. It is also difficult to apply the method to in-line-type assembly. A problem with density measurement, in turn, is that the measurement is highly sensitive to also other factors changing the concentration than just air, which is the most common gas in papermaking pulp.
The problem with microwave measurements carried out at two known pressures is that the pressures used in the measurements must be measured accurately from the very point where the microwave measurement is to be performed. The pressure indicator must be accurately calibrated in order to avoid systematic measurement errors in the gas content.
The problem with the arrangement disclosed in Finnish Patent 972,172 is that it is particularly difficult to accurately measure high gas contents.
An object of the invention is to provide a method and an arrangement implementing the method with which the aforementioned problems can be solved. This is achieved with a method of the type described in the introduction, which is characterized by measuring at least one variable of the microwave radiation after the radiation has propagated through the fluid, and determining the gas content of the fluid on the basis of changes caused by the gas bubbles in the variable of the microwave radiation.
The arrangement according to the invention is characterized in that the measuring arrangement comprises a gas measuring equipment for determining the gas content of the fluid on the basis of changes caused by the gas bubbles in the variable of the microwave signal.
The method and the arrangement according to the invention provide several advantages. The gas content of a fluid can be measured regardless of the consistency of the substance. The exact pressure value or even information about pressure variations is not required. Especially in measurements carried out in the paper industry, the measurement according to the invention is insensitive to other factors than the gas content.